Optimize strncat

Inline the strlen function to reduce the cost of function calls.

Issue: #I7L0VO
Test: libctest, benchmark
Signed-off-by: Nlitao <litao@kaihong.com>
Change-Id: I0d80a7c985392ebe860ba0deaa67ad7d093705c1

musl_src.gni

@@ -2204,6 +2204,7 @@ musl_src_porting_file = [
   "src/env/getenv.c",
   "src/string/strcasecmp.c",
   "src/string/strncasecmp.c",
+  "src/string/strncat.c",
   "src/string/stpncpy.c",
   "src/string/strncpy.c",
   "src/string/strcspn.c",

porting/linux/user/src/string/strncat.c

+#include <string.h>
+
+char *strncat(char *restrict d, const char *restrict s, size_t n)
+{
+	char *a = d;
+	while (*d != 0) d++;
+	while (n && *s) n--, *d++ = *s++;
+	*d++ = 0;
+	return a;
+}